1. Technical Field of the Invention
The present invention relates to the field of torque measurement and more particularly to a torque measuring method and apparatus, wherein temperature compensation is provided. The present invention is intended to provide a highly efficient and accurate method and apparatus for measuring the torque applied to a rotating shaft. The present invention has particular utility in measuring the torque applied to rotating shafts utilized in all sorts of machinery and particularly in aircraft engines.
2. Description of Background of Invention
In virtually every industrial application conceivable, rotating shafts are used to transmit and apply torque. In virtually all of these applications it is desired to be able to perform an accurate and efficient measurement of the torque applied to a rotating shaft. Such a torque measurement could be used to control or monitor the operation of the system in which the rotating shaft is utilized.
The purpose of such control and monitoring is quite varied. Most obviously, a measurement of the torque applied to a rotating shaft could indicate the approach of the applied torque to the design limit of the shaft and thus constitute an indicator of an impending catastrophic failure of the shaft under excessive torque. Similarly, measurement of the torque applied to a shaft can be an indication of the overstressing of the shaft and of the need to reduce the torque applied to the shaft. Overstressing of a shaft can also jeopardize the useful lifetime of the shaft by accelerating the fatigue failure of the shaft. Moreover, as the components of a torque transmission system wear with extended use, the torque necessary to drive the system changes. Thus, the variation in torque required to drive a system over an extended time period can be utilized as an indicator of excessive wear in portions of the power transmission system. For all of the above reasons and many others, it has long been known that accurate and efficient torque measurements are desirable in a wide range of industrial applications.
Indeed, the prior art contains numerous torque measuring devices applied to rotating shafts and utilizing various physical principles by which to measure torque. Thus, conventional torque sensors are known that use strain gauges as well as changes in the magnetic permeability of a material to measure torque. Differential optic sensing of torque is known, as are various capacitative mechanisms for measuring the torque applied to a shaft under load.
It is also well known that the application of torque to a shaft will result in a twisting deformation of the shaft. The amount of such twisting is directly related to the torque applied to the shaft by means of YOUNG'S modulus. Conventional torque measuring devices which measure the twist of a shaft or other drive member and utilize the measured twist as an indicator of the torque applied to the shaft or other member are also well known. However, as is well known, YOUNG'S modulus varies with changes in the temperature of the material. Thus, when measuring the twist to provide an accurate indication of the torque to which a shaft is subjected, if the torque is to be applied over a wide range of temperature, appropriate compensation must be provided for the change in YOUNG'S modulus due to temperature variation.
It has been known to provide a temperature sensor such as an RTD (resistance thermal device) near the twist sensor and to utilize the output from the RTD to provide a correction to the twist sensing-torque calculating mechanism to arrive at a temperature corrected value of the torque to which the rotating shaft is subjected. However, such a system suffers a serious drawback in that the location of the RTD is usually not identical with the location of the twist measuring sensor and thus inaccuracies in the temperature compensation are introduced. Moreover, the use of two sensing devices, i.e., the twist sensor and the thermal resistance device, increases the cost and complexity of the device.
In an environment such as a torque transmitting shaft in an aircraft engine, temperature variations are quite significant. At start-up, the ambient temperature of the shaft might be as low as -30.degree. C. while at operating conditions, the temperature of the shaft might well be in the range of 300.degree. C.-400.degree. C. This broad temperature variation range results in significant inaccuracies in the torque determination based on a sensing of shaft twist, since YOUNG'S modulus varies with the operating temperature. Accordingly, there has long been a need for a torque measuring device that provides a simple and accurate compensation for variations in the temperature of the shaft.
A known torque measurement device for a rotating shaft includes two gears placed adjacent to each other but fixed to the shaft a predetermined distance apart. Thus, the degree of twist in the shaft causes an angular displacement of one of the gears with respect to the other. Thus, the teeth of the two adjacently positioned intermeshed gears approach each other as the spacing therebetween changes in accord with the torque applied to the shaft. A single sensing device is positioned at the interface between the adjacent gears and detects the change in the spacing of the gear teeth, one from each gear, as they travel past the sensor. The output of the sensor is transmitted to an electronic processing circuit which compares the displacement of, or spacing between, each pair of teeth for each rotation of the shaft. The sensing device thus identifies the change in spacing of adjacent teeth. The distance between the adjacent teeth is related to the torque being transmitted by the shaft. Thus, by detecting the change in spacing of the teeth as they travel past the sensor, the torque applied to the shaft can be measured.